The invention relates to a device for quench propagation for a superconducting magnet. More particularly, the invention relates to a device for quench propagation for a superconducting magnet having at least one pair of superconducting coils, where in the event a first coil, which up to that point was superconducting, s becomes normal-conducting (quenches) where the quenching event occurs as a consequence of interference, the device causes the second coil of this pair to be converted from the superconducting operating state to the normal-conducting state with heating arrangements thermally connected to the coils.
In larger-sized superconducting magnets considerable quantities of energy are to be stored, which are, for example, in the MJ (mega-joule) range. In particular such magnets are strongly endangered in the event of an unintentional transition from the superconducting operating state to a normal-conducting state, even if this transition--also referred to as a "quench"--occurs initially only in one part of the magnet. Due to their low heat capacity, the superconducting coil conductors of the magnet, following a quench of one of the conductors, reach, very rapidly, high temperatures due to the resistance increase caused by the quench. Simultaneously, the specific resistance of the conductor also increases very quickly, which further increases the rate of heating. A consequence is excess voltages, which stress the insulation and, in the event of a flashover spark discharge, can lead to damage or destruction of the magnet.
Larger-sized superconducting magnets are frequently constructed as a combination of several superconducting coils, such as partial coils or partial windings. In order to protect these coils against damage or destruction through overheating, as through electrical flashovers, special protective measures are frequently provided. These measures may include, in particular, use of certain configurations for voltage limitation such as by bridging individual coils with protective resistors, as disclosed in German Patent Application No. DE-OS 23 01 152, semiconductor diodes, as disclosed in German Patent Application DE-OS 16 14 964, or with voltage limiters, as disclosed in German Patent Application No. DE-OS 17 64 369. In such a configuration however, in the event of a quench of one single coil, the currents of the coils of the magnet, generally connected in series, can assume widely differing patterns over time: the current of the quenched coil decreases in the process, while the current may increase in the neighbouring coils. In such a case, it is desirable to trigger cause a quench in additional or all coils of the magnet in order to de-excite the entire magnet or in order to uniformly distribute the magnetic energy converted into heat over the discrete coils. It may also be desired to favorably influence the current and field distribution during a coil quench. This is particularly important if the magnet includes a system of coils structured symmetrically in pairs. In the case of a coil pair the normally symmetric current distribution and, hence, also the field distribution, become asymmetric. The same is true of the interaction of the coils with their environment, in particular with induced eddy currents in a cryostat surrounding the coils or with ferromagnetic parts surrounding the coils such as, for example, iron screening. Under those conditions, considerable magnetic forces can occur stressing the cryogenic suspension of the coils.
Therefore, the intent is, in the event of a quench of one coil of one such a coil pair, to also trigger, as rapidly as possible, normal conductivity in the coil being symmetrical to it in order to permit the current distribution in the entire magnet and, consequently, also the force effects on the surrounding to become symmetrical. The net force on the coil pairs and on the cryostat components can in this way be reduced.
Measures for accelerating the propagation of normally conducting regions in a magnet comprising several coils are known. For example, the article "Some Basic Problems in Superconducting Magnet Design" in IEEE Transactions on Magnetics, Vol. MAG-17, No. 5, Sept. 1981, pages 1815-1822 discloses that a quench propagation can be increased through use of electrical heating elements on the coils, which are activated by a particular quench detector and fed by an external current supply.
For reasons of reliability, however, many times a "passive" quench propagation is desirable for a large superconducting magnet. A passive device is distinguished from an active device in that it exercises its voltage, temperature and force limiting functions without the use of and without the actuation of active elements such as, for example quench detectors, switches, and externally fed heating elements.
A passive quench propagation device is described, for example, in a paper entitled "Quenches in Large Superconducting Magnets" from the Proc. 6th Int. Conf. on Magnet Technology (MT-6), Bratislava (CSSR), Aug. 28-Sept. 2, 1977, pages 654 to 662. According to this device a superconducting magnet is to be wound on a coil former, which is a good electrical conductor such zs as highest grade aluminum, or on a secondary short circuit winding. In the event of a quench, the coil former or secondary short circuit winding takes over a part of the energy, functioning as transformer, and simultaneously heats the still superconducting parts of the magnet. In such a device, however, large losses of helium coolant occur in the process of informal excitation and de-excitation due to induced currents. If the rate of change is too high, then even the danger of triggering a quench unintentionally exists.
Another quench propagation device is disclosed in EP-B-0 115797. This quench spreading device works passively. This device, which is provided for a superconducting magnet with several discrete coils or partial coils, contains special heating arrangements in the form of films of normal-conducting material, which are connected in good heat-conducting contact with an associated coil. The operating voltages required in the event of a quench for these heating elements are tapped from a network of quench protecting resistors. In this case, however, the quench propagation from the quenched coil to the remaining coils takes place relatively slowly so that for a correspondingly long transition of time in the magnet correspondingly non-uniform conditions of current distribution and, hence, force effects can occur.
Corresponding magnets are applied, for example in the field of medical technology, as static ground field magnets in installations for nuclear spin tomography as disclosed in EP-B-0 011 335 or EP-A-0 056 691. Such ground field magnets contain in general several, for example four or six, annular superconducting single coils, which are arranged symmetrically in pairs with respect to a center plane. These superconducting single coils are expediently bridged with quench protective resistors or diodes. In that case, however, in the event of a quench, due to the non-symmetry of the single coil currents in the coils lying on both sides of the center plane, axial forces between these coils, cryogenic shields and possibly a present iron shielding can occur. These forces are, in a magnetic coil system for nuclear spin tomography, by far greatest at the front face single coils, since these coils in general have the greatest number of windings.